put it in your own words and Ill let you know if youre close to understanding.
The timekeeping element in every modern clock is a harmonic oscillator (a physical object that vibrates or oscillates at a particular frequency). The object can be a pendulum, a tuning fork, a quartz crystal, or the vibration of electrons in atoms as they emit microwaves. Atoms give off energy in packets that radiate at a very specific and consistent frequency. So, every atom is a clock because they are made up of particles that move in a systematic way.
Why do you think cesium 133 was the second obvious SI candidate defining the second?.
The technical explanation to why cesium 133 is the most accurate element to build an atomic clock is hard to picture but the interaction between the electron cloud and nucleus in cessium is more consistent than any other atom. So since the mechanism of an atomic clock is contained in the electron cloud there is a small range of error even in cessium.
(The math only works when we can see that a compilation of (X)femto seconds defines the age of a specific iron nickel meteorite containing U238)
And the point of that statement is?